In recent years, highly advanced illumination systems are being developed in order to enable consumers to obtain a desired ambiance for a particular room or space.
An example of such an illumination system is an arrangement of several illumination devices in a structure, such as a room, a lobby or a vehicle. The illumination devices, which may comprise light emitting diodes, emit visible light that carries codes (series of data symbols), i.e. coded light, identifying a light source or group of light sources. The light source codes are transmitted by modulating the luminance output from the illumination device. In such an illumination system, an optical receiver for receiving the codes is e.g. implemented in a remote control device for controlling the illumination devices or embedded in another device such as a switch or a sensor device.
Coded light also has advantages for mass-market illumination devices to perform e.g. fine tuning of the light sources and it allows embedding of individual information in the luminance output, representative of e.g. temperature or other status information.
Mass-market illumination devices may have a direct connection between an AC power source and the light sources. Such illumination devices typically do not contain an electrical driver. It is considered that such illumination devices should pave the road and generate a market for more advanced illumination systems.
Illumination devices wherein light sources are directly connected to an AC source have disadvantages when compared to DC-driven light sources (although these latter are more expensive), such as a lower efficacy and the production of some visible flicker resulting from the AC modulation frequency.
US 2006/175985 discloses a method and circuit for driving light emitting diodes (LEDs) in multiphase. A string of LEDs divided into groups connected to each other in series is provided. Each group is coupled to ground through separate conductive paths. A phase switch is provided in each conductive path. Increasing the input voltage turns on the string of LEDs, group by group in the sequence downstream the string.
An illumination device comprising such a circuit can be used to switch on/off respective (groups of) LEDs to obtain maximum luminance output as will be explained in more detail in the detailed description of the drawings.
A problem of such an AC-driven illumination device is that, while luminance output can be optimized, it fails to enable embedding of data symbols (codes) in the luminance output.